The present invention relates to a method and apparatus for shaping input waveform of power supply equipment for meeting harmonic current emissions standards. In particular, the present invention pertains to a passive component circuitry for shaping input current waveform for complying with harmonic current emissions regulations in low to medium power supply equipment.
Although harmonic currents emissions standards such as xe2x80x9cElectromagnetic Compatibility (EMC)xe2x80x94Part 3: Limitsxe2x80x94Section 2, Limits for harmonic current emissions (equipment input current xe2x89xa616 A per phase)xe2x80x9d, IEC 1000-3-2 Document, First Edition, 1995 (hereinafter IEC-1000-3-2) has been published some time ago, its impact on the ability of power supply manufacturers to export its low to medium range equipment has not been felt until 2000. For the first time, power supplies with input power exceeding 50 W are subjected to the IEC-1000-3-2 regulations in the Europe. A large variety of consumer electrical and electronic devices such as personal computers, printers, scanners and other accessories incorporate power supplies that come under the purview of IEC-1000-3-2.
It is well known in the field to rely on active power factor conversion or current conditioning to comply with IEC-1000-3-2. Prior art examples of active power factor conversion include U.S. Pat. Nos. 5,736,842 and 5,757,626. While active power factor conversion limits harmonic current emissions effectively, the power supplies are costly, complex and may be less reliable. Often the electromagnetic interference filters of active power factor conversion at the input stage are complicated.
U.S. Pat. No. 5,661,348 shows a typical example of current conditioning with inductors or chokes to meet limits imposed by lEC-1000-3-2. Inductors are simply and reliable devices but their bulk and weight impose considerable design constraints on modem consumer electrical or electronic devices that emphasize compactness and lightweight.
It is an object of the present invention to shape the input current waveform of a power supply so that it complies with harmonic current emission limits with passive components.
It is another object of the present invention to meet the harmonic current emission limits economically without relying on complex and unreliable filters and circuitry.
It is yet another object of the present invention to shape the input current waveform of a power supply in complying with harmonic current emission limits without paying excessive overhead for power factor conversion.
A passive component circuit comprising a bridge rectifier and is coupled in parallel to three capacitors; each of the first two capacitors is clamped in series by at least one diode and there in between is disposed another diode and a resistor for shaping the input current waveform such that it falls outside the Class D envelope of the limits on harmonic currents emissions. As such, power supplies incorporating such passive components meets automatically the criteria for Class A devices without further regards to power factor conversion and EMI filtering. Therefore, an economic strategy for complying with harmonic currents emissions limits is realised for low to medium power supplies.